Add user systemd units

The units can be used to automatically sync a folder in the users home dir
* every 3 minutes
* on local file changes (requires inotify-tools).

To enable the units for `~/google-drive/`:

```
systemctl --user enable grive-timer@google-drive.timer
systemctl --user start grive-timer@google-drive.timer
systemctl --user enable grive-changes@google-drive.service
systemctl --user start grive-changes@google-drive.service
```

The units can be enabled multiple times

CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 2.8)
 
+include(GNUInstallDirs)
+
 # Grive version. remember to update it for every new release!
 set( GRIVE_VERSION "0.5.1-dev" )
 
@@ -7,5 +9,7 @@ set( GRIVE_VERSION "0.5.1-dev" )
 add_definitions( -DVERSION="${GRIVE_VERSION}" )
 add_definitions( -D_FILE_OFFSET_BITS=64 -std=c++0x )
 
+add_subdirectory( systemd )
 add_subdirectory( libgrive )
 add_subdirectory( grive )
+	

systemd/CMakeLists.txt

@@ -0,0 +1,26 @@
+SET(GRIVE_SYNC_SH_BINARY "${CMAKE_INSTALL_PREFIX}/lib/grive/grive-sync.sh")
+
+CONFIGURE_FILE(grive-changes@.service.in grive-changes@.service @ONLY)
+CONFIGURE_FILE(grive-timer@.service.in grive-timer@.service @ONLY)
+
+install(
+  FILES
+    ${CMAKE_BINARY_DIR}/systemd/grive-changes@.service
+    ${CMAKE_BINARY_DIR}/systemd/grive-timer@.service
+  DESTINATION
+    lib/systemd/user
+)
+
+install(
+  FILES
+    grive-timer@.timer
+  DESTINATION
+    lib/systemd/user
+)
+
+install(
+  PROGRAMS
+    grive-sync.sh
+  DESTINATION
+    lib/grive
+)

systemd/grive-changes@.service.in

@@ -0,0 +1,11 @@
+[Unit]
+Description=Google drive sync (changed files)
+
+[Service]
+ExecStart=@GRIVE_SYNC_SH_BINARY@ listen "%i"
+Type=simple
+Restart=always
+RestartSec=30
+
+[Install]
+WantedBy=default.target

systemd/grive-sync.sh

@@ -0,0 +1,118 @@
+#!/bin/bash
+
+# Copyright (C) 2009 Przemyslaw Pawelczyk <przemoc@gmail.com>
+#           (C) 2017 Jan Schulz <jasc@gmx.net>             
+##
+## This script is licensed under the terms of the MIT license.
+## https://opensource.org/licenses/MIT
+
+# Fail on all errors
+set -o pipefail
+
+# We always start in the current users home directory so that names always start there
+cd ~
+
+
+### ARGUMENT PARSING ###
+SCRIPT=${0}
+
+if [[ -z ${2} ]] || [[ ! -d ${2} ]] ; then
+	echo "Need a directory name in the current users home directory as second argument. Aborting."
+	exit 1
+fi
+
+DIRECTORY=$2
+
+if [[ -z ${1} ]] ; then
+	echo "Need a command as first argument. Aborting."
+	exit 1
+else
+	if [[ "sync" == "${1}" ]] ; then
+		COMMAND=sync
+	elif [[ "listen" == "${1}" ]] ; then
+		COMMAND=listen
+	else
+		echo "Unknown command. Aborting."
+		exit 1
+	fi
+fi
+
+
+### LOCKFILE BOILERPLATE ###
+LOCKFILE="/run/user/$(id -u)/$(basename $0)_${DIRECTORY//\//_}"
+LOCKFD=99
+
+# PRIVATE
+_lock()             { flock -$1 $LOCKFD; }
+_no_more_locking()  { _lock u; _lock xn && rm -f $LOCKFILE; }
+_prepare_locking()  { eval "exec $LOCKFD>\"$LOCKFILE\""; trap _no_more_locking EXIT; }
+
+# ON START
+_prepare_locking
+
+# PUBLIC
+exlock_now()        { _lock xn; }  # obtain an exclusive lock immediately or fail
+exlock()            { _lock x; }   # obtain an exclusive lock
+shlock()            { _lock s; }   # obtain a shared lock
+unlock()            { _lock u; }   # drop a lock
+
+### SYNC SCRIPT ###
+# Idea: only let one script run, but if the sync script is called a second time 
+# make sure we sync a second time, too
+
+sync_directory() {
+	_directory=${1}
+	if ping -c1 -W1 -q accounts.google.com >/dev/null 2>&1; then
+	    true
+	    # pass
+	else
+	    echo "Google drive server not reachable..."
+	    exit 0
+	fi
+
+	reset_timer_and_exit() { echo "Retriggered google drive sync" && touch -m $LOCKFILE && exit; }
+
+	exlock_now || reset_timer_and_exit
+
+	TIME_AT_START=0
+	TIME_AT_END=1
+	while [[ ${TIME_AT_START} -lt ${TIME_AT_END} ]]; do
+	    echo "Syncing ${_directory}..." 
+	    TIME_AT_START=$(stat -c %Y $LOCKFILE)
+	    grive -p ${_directory} 2>&1 | grep -v -E "^Reading local directories$|^Reading remote server file list$|^Synchronizing files$|^Finished!$"
+	    TIME_AT_END=$(stat -c %Y $LOCKFILE)
+	    echo "Sync of ${_directory} done." 
+	done
+
+	# always exit ok, so that we never go into a wrong systemd state
+	exit 0
+}
+
+### LISTEN TO DIRECTORY CHANGES ###
+
+
+listen_directory() {
+	_directory=${1}
+
+	type inotifywait >/dev/null 2>&1 || { echo >&2 "I require inotifywait but it's not installed. Aborting."; exit 1; }
+
+	echo "Listening for changes in ~/${_directory}"
+
+	while true #run indefinitely
+	do 
+		# Use a different call to not need to change exit into return
+		inotifywait -q -r -e modify,attrib,close_write,move,create,delete --exclude ".grive_state|.grive" ${_directory} > /dev/null 2>&1 && ${SCRIPT} sync ${_directory}
+	done
+
+	# always exit ok, so that we never go into a wrong systemd state
+	exit 0
+}
+
+if [[ ${COMMAND} == listen ]] ; then
+	listen_directory ${DIRECTORY}
+else
+	sync_directory ${DIRECTORY}
+fi
+
+# always exit ok, so that we never go into a wrong systemd state
+exit 0

systemd/grive-timer@.service.in

@@ -0,0 +1,6 @@
+[Unit]
+Description=Google drive sync 
+After=network-online.target
+
+[Service]
+ExecStart=@GRIVE_SYNC_SH_BINARY@ sync "%i"

systemd/grive-timer@.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Google drive sync (fixed intervals)
+
+[Timer]
+OnCalendar=*:0/5
+OnBootSec=3min
+OnUnitActiveSec=5min
+Unit=grive-timer@%i.service
+
+[Install]
+WantedBy=timers.target